Lego dimensions - worth it for my son and should I get a PS4 for it?

PS3 is a dead system. Nothing new is coming out for it. If you only want to play older games its fine though.

Lego Dimensions is an awesome game and I love playing it with my kids. However, all indications are that it is up for cancellation after this holiday season. The third series of character releases should have started months ago. They pushed the series 2 release table back so some new characters would come out for Christmas. Lego then "announced" characters we knew about before last holiday season at E3. They also patched out some levels that will no longer be released. All that is just to say it is highly unlikely that you'll need a PS4 for Lego Dimenions. That does make this a great time for your kid to be getting into this game, because those game figures will likely be going on sale next year as stores try to get rid of it. Same thing happened with Disney Infinity this year and we loaded up. Even the big 3 packs were going for like $5-$10.

Maybe they'll release a Lego Dimensions 2 (unlikely), and maybe you'd need a PS4 for that, but I doubt a sequel would connect to the old game. The characters you buy should still work on the new system though. Also, if you were to hold off and upgrade later, you would only need to buy the PS4 game disc and maybe a portal to play on the PS4. The character pieces are compatible with all formats.